This invention relates, in general, to a dual spark ignition internal combustion engine in which two spark plugs are disposed in each combustion chamber to ignite the air-fuel mixture inducted thereto, and more particularly to an improvement in the ignition system of the above-mentioned engine.
In connection with the exhaust gas emission control of a spark-ignition internal combustion engine which discharges exhaust gases containing nitrogen oxides (NOx), it is difficult to decrease the emission level of NOx because the formation of NOx is increased as the combustion is improved, i.e. combustion temperature rises, and NOx once generated in the combustion chamber is not easily removed by a catalytic reduction reaction, the catalyst also producing problems with respect to performance and durability. Therefore, the greatest effort is now directed to suppression of the NOx generation in the combustion chamber. Since the NOx emission control downstream of the combustion chamber encounters the above-mentioned problems, it is necessary to achieve suppression of NOx generation within the combustion chamber. For this purpose, it has been proposed to supply exhaust gases into the combustion chamber in order to lower the maximum temperature of combustion carried out in the combustion chamber. This is achieved, for example, by a so-called exhaust gas recirculation system (EGR system) which is known as disclosed, for example, in U.S. Pat. No. 3,756,210, issued Sept. 4, 1973 to Kuehl. With this recirculation of the exhaust gases, the emission level of NOx is found to decrease as the amount of the exhaust gases is increased. However, by supplying the combustion chamber with a considerable proportion of the exhaust gases, the combustion time of the air-fuel mixture is increased and therefore stable and smooth combustion of the air-fuel mixture in the combustion chamber fails. In view of the above, the amount of the exhaust gases supplied to the combustion chamber is restricted to a relatively low level in due consideration of both stable combustion and NOx generation control. The unstable combustion of the air-fuel mixture causes deterioration of engine power output and fuel consumption characteristics.
In view of the above, attention has been directed to the idea that stable combustion in the combustion chamber is obtained by fast burn of the air-fuel mixture in the combustion chamber by shortening the combustion time of the air-fuel mixture. To this end, a dual spark plug ignition engine with two spark plugs in each combustion chamber has been proposed by the same applicant as the present application to maintain stable combustion in the combustion chamber even though a considerably large amount of exhaust gases is recirculated back to the combustion chamber.
However, this proposed dual spark plug ignition engine requires further improvement from the standpoint of decreasing engine noise and increasing engine durability, since the engine noise is increased and the engine durability is decreased by an excessively high pressure rise in the combustion chamber of the engine under a high power output engine operating condition. The excessively high pressure rise occurs under the high power output engine operating conditions because of the following reasons: (1) The exhaust gas recirculation is stopped or controlled to its minimum value to prevent degradation of engine power output and fuel consumption; (2) The air-fuel ratio of the mixture supplied to the combustion chamber is slightly enriched to obtain high power output; and (3) Since the throttle valve is widely opened, the charging efficiency of the inducted air-fuel mixture becomes considerably high.